Embodiments of the present invention relate to a thin film transistor liquid crystal display (TFT-LCD) and a method of manufacturing the same.
Thin film transistor liquid crystal displays (TFT-LCDs) have the advantages of small volume, low energy consumption, low radiation, etc., and are gradually prevailing in the market of flat plate displays. A TFT-LCD is mainly constituted by an array substrate and a color filter substrate assembled to face to each other. A plurality of thin film transistors and a plurality of pixel electrodes are arranged on the array substrate in matrix, and each of the pixel electrodes is controlled by a thin film transistor. The pixel electrode is charged when the thin film transistor is turned-on. After charging, the voltage of the pixel electrode remains unchanged until the next time of turn-on of the thin film transistor.
Because of parasitic capacitance between the drain electrode and the gate electrode of the thin film transistor, a kickback voltage ΔVp is generated at the moment the charging of the pixel electrode is finished. The kickback voltage ΔVp is expressed as follows:
            Δ      ⁢                          ⁢              V        p              =                  (                              V            gh                    -                      V            gl                          )            ⁢                        C          gd                                      C            gd                    +                      C            lc                    +                      C            s                                ,where Vgh is the turn-on voltage of the gate electrode, Vgl is the turn-off voltage of the gate electrode, Clc is the liquid crystal capacitance of the pixel, Cgd is the parasitic capacitance, and Cs is the storage capacitance. Researches have shown that the polarity of the pixel electrode can vary due to the kickback voltage ΔVp by the parasitic capacitance, and thus the voltage difference between the positive and the negative polarities varies, which causes flickers on the displayed image and thus severe deterioration of the display quality.
Presently, in forming a TFT-LCD array substrate, a wet etching process is typically employed to form the gate electrodes of thin film transistors. Since the gate electrodes are made by a metal film with a relatively large thickness (such as 3000-6000 Å) and the wet etching process shows a relatively poor uniformity, the inclination angles of side surfaces of the gate electrodes as well as the areas occupied by the side surfaces vary among different positions on same one mother substrate. One side surface of each gate electrode is positioned within the overlapping region between the gate electrode and the corresponding drain electrode. When the inclination angle of the side surface of the gate electrode is small, the overlapping area between the drain electrode and the gate electrode becomes large; when the inclination angle of the side surface of the gate electrode is large, the overlapping area between the drain electrode and the gate electrode becomes small. That is, the inclination angle of the side surface of the gate electrode imposes a direct influence on the overlapping area between the drain electrode and the gate electrode. In addition, according to the formula defining the capacitance, the parasitic capacitance is directly proportional to the above-described overlapping area. Therefore, in the case that the inclination angles of side surface of the gate electrodes vary among different positions of the mother substrate, the parasitic capacitance of the thin film transistors among the different positions varies, and accordingly the kickback voltages ΔVp of the thin film transistors also vary among the different positions of the mother substrate. Furthermore, in forming the liquid crystal panel of a large area, both the area of the mother substrate and the thickness of the gate electrodes are large, and thus the variation of the inclination angles of side surface among the gate electrodes becomes more considerable over the mother substrate. Therefore, the variation of the parasitic capacitance among the thin film transistors is aggravated, and accordingly the variation among the kickback voltages ΔVp of the thin film transistors is also aggravated. In the case that the variation among the kickback voltages ΔVp is aggravated, the adjustment by a drive circuit becomes more difficult, which may cause more flickers on the displayed image and more severe deterioration of the display quality.